Flip chip packages include a semiconductor die mounted onto a package substrate with the active side of the die facing the substrate. Conventionally, interconnection of the circuitry in the die with circuitry in the substrate is made by way of bumps which are attached to an array of interconnect pads on the die, and bonded to a corresponding (complementary) array of interconnect pads (often referred to as “capture pads”) on the substrate.
The areal density of electronic features on integrated circuits has increased enormously, and chips having a greater density of circuit features also may have a greater density of sites for interconnection with a package substrate.
The package is connected to underlying circuitry, such as a printed circuit board (e.g., a “motherboard”) in the device in which it is employed, by way of second level interconnects (e.g., pins, solder balls) between the package and the underlying circuit. The second level interconnects have a greater pitch than the flip chip interconnects, and so the routing on the substrate conventionally “fans out”. Significant technological advances have enabled construction of fine lines and spaces; but in the conventional arrangement space between adjacent pads limits the number of traces than can escape from the more inward capture pads in the array, and the fan out routing between the capture pads beneath the die and the external pins of the package is conventionally formed on multiple metal layers within the package substrate. For a complex interconnect array, substrates having multiple layers may be required to achieve routing between the die pads and the second level interconnects on the package.
Multiple layer substrates are expensive, and in conventional flip chip constructs the substrate alone typically accounts for more than half the package cost (about 60% in some typical instances). The high cost of multilayer substrates has been a factor in limiting proliferation of flip chip technology in mainstream products.
In conventional flip chip constructs the escape routing pattern typically introduces additional electrical parasitics, because the routing includes short runs of unshielded wiring and vias between wiring layers in the signal transmission path. Electrical parasitics can significantly limit package performance.
In some conventional processes, flip chip interconnection is made by contacting the bumps or balls on the die with corresponding interconnect sites on the substrate circuitry, and then heating to reflow the fusible portion of the solder bumps (or to reflow the solder bumps in their entirety) to make the electrical connection. In such processes the melted solder may flow from the interconnect site along the metal of the circuitry, depleting the solder at the connection site; and where the bumps are collapsible under reflow conditions the bumps may contact adjacent circuitry or nearby bumps, resulting in electrical failure. To avoid these problems, typically in conventional flip chip packages the solder is confined by a “solder mask”, consisting of a layer of dielectric material overlying the patterned metal layer at the die mount surface of the substrate, and having openings each exposing an interconnect site on the underlying circuitry. Process limitations in patterning the solder mask prevent reliably forming well-aligned and consistently dimensioned openings and, accordingly, where a solder mask is employed, substrates having fine circuitry feature dimensions as would be required for finer pitch interconnection are not attainable.
The interconnect pitch in conventional flip chip interconnects is limited in part by the dimensions of the capture pads on the substrate (typically the capture pads are much wider than the circuit elements connecting them). Recently flip chip substrate circuitry design has been disclosed, in which reliable interconnection is made on narrow circuit elements on the substrate, as for example in “bond-on-narrow pad interconnections” (BONP), as described generally in co-pending U.S. application Ser. No. 11/388,755, filed Mar. 26, 2006; and as for example in “bump-on-lead interconnections” (BOL), as described generally in co-pending U.S. application Ser. No. 110/985,654, filed Nov. 10, 2004, both incorporated herein by reference. Where a conventional solder mask is to be employed, limitations in the process for patterning the solder mask can limit pitch reduction even in some BONP or BOL substrate configurations. The exposed bondable surface of the lead may be contaminated by or covered by solder mask residue, resulting in an imperfect solder joint; or, the bondable surface of the lead may be inconsistently or only partially exposed at the interconnect site, resulting in an unreliable and inconsistent trace structure.